SQL选择不同的列值

Question

Lets say I have a MySQL table that has the following entries;

ID,     PRODUCTID
95,     2
95,     2
95,     1
96,     1
102,    1
102,    5
95,     5

How do I select the PRODUCTID(s) that only have one distinct ID-column value.

The result I want to get back is:

PRODUCTID
2

I have tried something in the lines of;

SELECT DISTINCT PRODUCTID
FROM `TABLE`
GROUP  BY ID, PRODUCTID
HAVING COUNT(*) = 1;

Answer 1

Group by productid and then use a count(distinct id) to count unique id s

SELECT PRODUCTID
FROM your_table
GROUP BY PRODUCTID
HAVING COUNT(distinct ID) = 1

Answer 2

SELECT DISTINCT x.PRODUCTID 
           FROM my_table x 
           LEFT 
           JOIN my_table y 
             ON y.PRODUCTID = x.PRODUCTID 
            AND y.ID <> x.ID 
          WHERE y.ID IS NULL;

Answer 3

COUNT(DISTINCT) is one way to solve this problem. However, I prefer to compare MIN() and MAX() -- because these are cheaper to calculate:

SELECT PRODUCTID
FROM `TABLE`
GROUP BY PRODUCTID
HAVING MIN(ID) = MAX(ID);

If ID could be NULL , you would need to add another condition:

SELECT PRODUCTID
FROM `TABLE`
GROUP BY PRODUCTID
HAVING (MIN(ID) = MAX(ID) AND COUNT(*) = COUNT(ID)) OR  -- No NULL values
        (MIN(ID) IS NULL);                              -- or all NULL values

Answer 4

ID,     PRODUCTID
95,     2
95,     2
95,     1
96,     1
102,    1
102,    5
95,     5
you can use this this is working fine
SELECT distinct product_id from  category GROUP  BY id, product_id
HAVING COUNT(*) = 1;